Directly store the number of documents in an index

src/index.rs

@@ -10,7 +10,7 @@ use roaring::RoaringBitmap;
 use crate::facet::FacetType;
 use crate::fields_ids_map::FieldsIdsMap;
 use crate::{default_criteria, Criterion, Search};
-use crate::{BEU32, DocumentId, FieldId, ExternalDocumentsIds};
+use crate::{BEU32, BEU64, DocumentId, FieldId, ExternalDocumentsIds};
 use crate::{
     RoaringBitmapCodec, BEU32StrCodec, StrStrU8Codec, ObkvCodec,
     BoRoaringBitmapCodec, CboRoaringBitmapCodec,
@@ -19,6 +19,7 @@ use crate::{
 pub const CRITERIA_KEY: &str = "criteria";
 pub const DISPLAYED_FIELDS_KEY: &str = "displayed-fields";
 pub const DOCUMENTS_IDS_KEY: &str = "documents-ids";
+pub const NUMBER_DOCUMENTS_IDS_KEY: &str = "number-documents-ids";
 pub const FACETED_DOCUMENTS_IDS_PREFIX: &str = "faceted-documents-ids";
 pub const FACETED_FIELDS_KEY: &str = "faceted-fields";
 pub const FIELDS_IDS_MAP_KEY: &str = "fields-ids-map";
@@ -101,12 +102,25 @@ impl Index {
 
     /// Writes the documents ids that corresponds to the user-ids-documents-ids FST.
     pub fn put_documents_ids(&self, wtxn: &mut RwTxn, docids: &RoaringBitmap) -> heed::Result<()> {
-        self.main.put::<_, Str, RoaringBitmapCodec>(wtxn, DOCUMENTS_IDS_KEY, docids)
+        let len = docids.len();
+        self.main.put::<_, Str, RoaringBitmapCodec>(wtxn, DOCUMENTS_IDS_KEY, docids)?;
+        self.main.put::<_, Str, OwnedType<BEU64>>(wtxn, NUMBER_DOCUMENTS_IDS_KEY, &BEU64::new(len))?;
+        Ok(())
     }
 
     /// Returns the internal documents ids.
     pub fn documents_ids(&self, rtxn: &RoTxn) -> heed::Result<RoaringBitmap> {
-        Ok(self.main.get::<_, Str, RoaringBitmapCodec>(rtxn, DOCUMENTS_IDS_KEY)?.unwrap_or_default())
+        let documents_ids = self.main.get::<_, Str, RoaringBitmapCodec>(rtxn, DOCUMENTS_IDS_KEY)?
+            .unwrap_or_default();
+        Ok(documents_ids)
+    }
+
+    /// Returns the number of documents indexed in the database.
+    pub fn number_of_documents(&self, rtxn: &RoTxn) -> anyhow::Result<u64> {
+        let count = self.main.get::<_, Str, OwnedType<BEU64>>(rtxn, NUMBER_DOCUMENTS_IDS_KEY)?
+            .map(BEU64::get)
+            .unwrap_or_default();
+        Ok(count)
     }
 
     /* primary key */
@@ -251,6 +265,11 @@ impl Index {
         }
     }
 
+    /// Retrieve the number documents ids that faceted under this field id.
+    pub fn number_faceted_documents_ids(&self, rtxn: &RoTxn, field_id: FieldId) -> heed::Result<u64> {
+        todo!()
+    }
+
     /* criteria */
 
     pub fn put_criteria(&self, wtxn: &mut RwTxn, criteria: &[Criterion]) -> heed::Result<()> {
@@ -301,11 +320,6 @@ impl Index {
         Ok(documents)
     }
 
-    /// Returns the number of documents indexed in the database.
-    pub fn number_of_documents(&self, rtxn: &RoTxn) -> anyhow::Result<usize> {
-        Ok(self.documents_ids(rtxn).map(|docids| docids.len() as usize)?)
-    }
-
     pub fn search<'a>(&'a self, rtxn: &'a RoTxn) -> Search<'a> {
         Search::new(rtxn, self)
     }

src/update/clear_documents.rs

@@ -11,7 +11,7 @@ impl<'t, 'u, 'i> ClearDocuments<'t, 'u, 'i> {
         ClearDocuments { wtxn, index }
     }
 
-    pub fn execute(self) -> anyhow::Result<usize> {
+    pub fn execute(self) -> anyhow::Result<u64> {
         let Index {
             env: _env,
             main: _main,

src/update/delete_documents.rs

@@ -46,7 +46,7 @@ impl<'t, 'u, 'i> DeleteDocuments<'t, 'u, 'i> {
         Some(docid)
     }
 
-    pub fn execute(self) -> anyhow::Result<usize> {
+    pub fn execute(self) -> anyhow::Result<u64> {
         // We retrieve the current documents ids that are in the database.
         let mut documents_ids = self.index.documents_ids(self.wtxn)?;
 
@@ -242,6 +242,6 @@ impl<'t, 'u, 'i> DeleteDocuments<'t, 'u, 'i> {
 
         drop(iter);
 
-        Ok(self.documents_ids.len() as usize)
+        Ok(self.documents_ids.len())
     }
 }